Florida Geological Survey
- Hazards - Sinkholes
Sinkholes are a fact of life in Florida. They occur because the entire state of Florida is underlain by limestone,
a type of rock that is slowly dissolved by weak natural acids found in rain and in the pore spaces in soil. The formation of sinkholes is explained in
detail in the Geology Topics section of our website.
There are more sinkholes in some parts of Florida than in others. Map Series 110, which may be accessed from this website, depicts
four types of sinkhole terrain in Florida. Where limestone bedrock is thinly covered, depending on the area, there are relatively few to a moderate
number of sinkholes. They are usually shallow and broad and develop gradually and are called “solution sinkholes.” In other areas the limestone is
covered by sand that is easily permeable to water and incohesive because it contains little clay. Sinkholes in these areas tend to be few and small
and develop gradually even though the sand may range in thickness from 30 to 200 feet. They are referred to as “cover-subsidence sinkholes.”
Sinkholes become more of a problem in areas where sediments that lie above the limestone are mainly clays mixed with sand.
Clay causes these sediments, which also range in thickness from 30 to 200 feet, to be cohesive. They are not very permeable to water. In
these areas sinkholes are most numerous. They vary in size and may form suddenly. In a few areas of Florida over 200 feet of sediments cover
the underlying limestone. These sediments are cohesive because of the clay and layers of limestone they contain. Although there are not many
sinkholes in these areas, the ones that occur are deep and wide. These types of sinkholes are referred to as “cover-collapse sinkholes” because
cohesive layers of sediment collapse into underground cavities when they form.
The abrupt formation of sinkholes may follow extreme rain producing events such as tropical storms or hurricanes. This is because
the weight of a large amount of rain water at the earth’s surface may bring about the collapse of an underground cavity if its limestone “ceiling”
has become thin. This tendency for sinkholes to form following events that produce large amounts of rainfall is made worse in times of drought.
During periods of drought underground cavities that might normally be filled with water may be only partially filled. These cavities are less likely
to bear the weight of flood waters without collapsing.
Sinkholes are part of Florida’s natural environment. Surface water flows through sinkholes into the underlying limestone layers
of the Floridan aquifer system, the major source of drinking water in Florida. Because they provide a direct connection from the surface to the
aquifer system, it is very important to protect sinkholes from pollution. This may be as simple as making sure that sinkholes do not become convenient
sites for dumping. The use of conservation buffers (strips of land maintained in permanent vegetation) around sinkholes can help protect groundwater
from fertilizers and pesticides that might otherwise enter sinkholes on farm lands. The use of fencing around sinkholes may help protect groundwater
from animal waste.
People who live in Florida or who are moving into the state should be aware of sinkholes and the possible problems they pose for
homeowners. The section of this website that addresses frequently asked questions concerning sinkholes contains useful information on practical issues
associated with sinkholes. Topics important to homeowners and prospective homeowners, such as sinkhole disclosure laws and insurance issues, are covered.
These links will provide additional information concerning sinkholes in Florida:
Click on any picture to enlarge.
